DESCRIPTION Following a neurological injury such as a stroke, functional mobility is often limited. One potential cause of reduced mobility is decreased gait stability, as evidenced by the increased risk of falls after a stroke. A second potential cause of reduced mobility is an increased energetic cost of walking, which in combination with reduced cardiovascular capacity can lead to activity-limiting fatigue. This project proposes that both the decreased gait stability and increased energetic cost seen after a stroke can be partially attributed to altered sensorimotor integration, as indicated by a decreased capacity to accurately control voluntary movement. Accurate motor control requires the ability to produce the intended muscle activation pattern (actuation accuracy) and the ability to sense the mechanical state of the moving body segment using feedback from the periphery (sensation accuracy), both abilities which are commonly reduced after a stroke. The proposed experiments will test whether reductions in control accuracy affect stability and energetic cost, based on the predictions of mechanical models. The first objective of the proposed project is to identify the effects of reduced control accuracy on lateral gait stability following a stroke. Simple mechanical models predict that sagittal plane gai stability can be maintained passively in response to small perturbations, but frontal plane stability requires active control. The simplest control strategy to maintain lateral stability is t choose an appropriate mediolateral foot placement of the swing leg, with more lateral foot placement requiring less accurate control. The proposed experiments will test whether decreased control accuracy explains altered frontal plane mechanics following a stroke. The primary anticipated result of these experiments is that delivering enhanced sensory feedback to persons who have experienced a stroke will restore a more typical gait pattern, a finding with clear clinical implications. The second objective of the proposed project is to quantify the contribution of limited control accuracy to the increased energetic cost of movement following a stroke. In typical gait, energetic economy is improved by storing and returning mechanical energy in the elastic Achilles tendon, allowing strong push-off without requiring large amounts of plantarflexor muscle work. Similarly, the energetic demand of bouncing can be substantially reduced by taking advantage of tendon elasticity, while the simplicity of the task in comparison to walking eases quantification of system mechanics. The proposed experiments will quantify the effect on bouncing efficiency of: 1) altered mechanical tissue properties, specifically reduced tendon stiffness; 2) changes in efficiency of the muscular conversion of metabolic energy to mechanical energy; 3) an inability to identify the optimal movement pattern using sensory feedback. The primary anticipated result is that following a stroke, patients will be unable to identify the pattern of movement that takes optimal advantage of system mechanics. By choosing a non-optimal movement pattern, energetic cost will be increased.